1. Field of the Invention
The present invention relates to a method for recording information on a phase-change optical recording medium.
2. Description of the Related Art
In optical storage media such as DVDs (Digital Versatile Discs), write data is modulated into a length of a record mark along the track of the recording layer and then recorded with the length of a record mark associated with a time length of the write data (an integral multiple of T or nT), where T is the write channel clock cycle. For example, CD-RWs (Compact Disc Rewritable) employ the EFM modulation scheme, so that data to be written is modulated into a record mark having any one of lengths 3T to 11T for recording.
Those optical storage media as mentioned above often employ phase-change film for the recording layer, in the case of which the record mark portion has the phase-change film in an amorphous-state, and crystalline-state space portions are formed between each record mark.
To form the amorphous-state record mark portion, a crystalline portion is melted with a laser beam and then rapidly cooled. To form the crystalline-state portion, the phase-change film is irradiated with a laser beam in a manner such that the film is held for a certain period of time or longer at the crystallization temperature thereof or higher.
To achieve the amorphous-state, the phase-change film is irradiated with a laser beam, which includes a pulse train of a write pulse at write power and a cooling pulse generally at bottom power, to be thereby melted and then rapidly cooled. On the other hand, to achieve the crystalline-state, the phase-change film is irradiated with a laser beam at an erase power level. As a method for recording information on an optical recording medium, known is the mark edge recording scheme which is adapted for higher densities. To record information on a phase-change optical recording medium using the mark edge recording scheme, write and cooling pulses having a pulse width of 0.5T with respect to the write channel clock cycle T are employed.
In this case, each time the time length of write data is incremented by 1T, a set of a write pulse and a cooling pulse is to be added. Such a multi-pulse write waveform allows write data of different time lengths to be recorded always in constant heating and cooling conditions, thereby reducing an edge shift which is dependent on the time length of write data. Furthermore, to perform high-speed write operations using such a write waveform, the write channel clock frequency is made higher, e.g., two times or four times, at the same rate as the write linear speed, with the write waveform kept unchanged.
Thus, high-speed write operations may be performed by making the write channel clock frequency higher, e.g., two times or four times, with the write waveform kept unchanged. However, this results in the width of the write pulse and the cooling pulse being significantly reduced. It is thus difficult to attain the temperature and the cooling speed that are required to change the phase of the recording film. This causes a record mark to be insufficiently formed, thus raising a problem that a record mark of a correct mark length cannot be obtained.
Furthermore, in performing high-speed write operations, the turn-on time or the turn-off time of the semiconductor laser drive circuit may increase with respect to the write channel clock. In this case, for example, a 9T record mark may have a distorted write waveform when recorded. This will thus raise a problem that it is difficult to sufficiently heat and cool the phase-change optical recording medium, leading to a shorter record mark.
In contrast to this, according to the invention disclosed in Japanese Patent Laid-Open Publication No. 2002-288837, a record mark is formed using write laser light as multi-pulse light. This multi-pulse includes at least a leading write pulse and a trailing cooling pulse. A cooling pulse and a write pulse are inserted between the leading write pulse and the trailing cooling pulse as the time length of write data increases.
The set of cooling and write pulses to be inserted are adapted such that one additional set is inserted each time the data length of write data is increased by 2T in each case of n of the time length nT of write data being an odd number and an even number.
Here, as means for increasing the storage capacity of an optical recording medium, there is available a method of employing a multi-layered optical recording medium which has recording layers stacked in a plurality of layers. In such a multi-layered optical recording medium, a recording layer on which light is incident needs to be thin enough to be semitransparent to light passing therethrough. It is thus difficult to provide a thick heat dissipating layer for dissipating heat generated in the recording layer when irradiated with write laser light. The recording layer will be thus configured to be only gradually cooled, but rapidly cooled or rapidly heated with great difficulty.
In such a recording layer configured to be gradually cooled, recrystallization will readily occur from a melted interface portion of the recording film when a record mark is formed. In particular, a recording film suitable to high-speed recording needs to be crystallized at a high speed, and will be thus also recrystallized easily in nature. As is known in the art, this recrystallization will cause an amorphous-state record mark to be contracted and a read signal to be less amplitude modulated. Additionally, this recrystallization will also cause noticeable recrystallization at the front edge portion of a record mark, and thus a change in the shape of the front edge portion results in a significant degradation in jitter characteristic of the read signal.
As a countermeasure against the recrystallization of record marks, disclosed in Published Japanese translation No. 2004-520675 of PCT application is an improved write pulse strategy for reducing heat accumulation in the recording film. According to this improvement, a record mark of nT is formed using m write pulses, where m is less than n, such that a longer cooling time between pulses is allowed to facilitate heat dissipation from the recording film and thus reduce heat accumulation.
Furthermore, the write strategy (2T strategy) disclosed in Japanese Patent Laid-Open Publication No. 2002-288837 mentioned above is to form a record mark of nT using n/2 write pulses, and is thus covered by the write strategy disclosed in Published Japanese translation No. 2004-520675 of PCT application. However, the 2T strategy is effective at recording information on a multi-layered and high-speed recording optical recording medium.
On the other hand, as is known in the art, after an optical recording medium having a record mark formed thereon is held at a high temperature, overwriting a pre-storage record mark would cause jitter characteristics to degrade during reading. In contrast to this, the invention disclosed in Japanese Patent Laid-Open Publication No. Hei 11-167722 suggests that the post-storage write linear speed should be made slower than the optimal pre-storage write linear speed, thereby overwriting in an improved manner. However, in the case of high-speed recording, the optimal speed of writing on the recording film itself needs to be increased, and in consideration of balance between the write speed and the archiving characteristics, it is difficult to slow the post-storage write linear speed.
Using the 2T strategy as described above, information was read and written, for example, on a thin light-incident recording layer of a two-layered optical recording medium, and it was found that a good jitter characteristic was not obtained at the front edge of the record mark. Likewise, it was also found that performing read and write operations on an optical recording medium, after it has been written using the 2T strategy and held at a high temperature, would cause a further degradation in the jitter characteristic of the front edge portion of the record mark.